1. Field of the Invention
The present invention relates to a filter circuit such as a low pass filter or high-pass filter that makes it possible to perform cutoff frequency adjustment easily and to a method of measuring the cutoff frequency of the filter circuit.
2. Description of the Related Art
Varied signal processing such as high-frequency noise removal may be performed by installing a low pass filter or high pass filter, or the like, in an integrated circuit. Low pass filters and high pass filters have a cutoff frequency Fc corresponding with the characteristic value of the impedance element that constitutes the filter. The cutoff frequency Fc is dependent on the capacitance value or resistance value of the impedance element that constitutes the filter circuit. Meanwhile, the circuit elements of the integrated circuit are subject to the influence of structural variations and therefore fluctuations in the characteristic value are unavoidable. Therefore, the filter circuit built in the integrated circuit requires a circuit for tuning the cutoff frequency Fc. For example, Japanese Patent Application Laid Open No. H6-251303, such as FIGS. 1 and 5 and paragraph [0035], for example, describes the setting of the cutoff frequency of a bandpass filter or low pass filter, or the like.
A conventional, general cutoff frequency setting method entails supplying a sine wave with a predetermined frequency to the input of the filter, checking the relationship between the predetermined frequency and the cutoff frequency of the filter by monitoring the output signal of the filter, and then tuning the filter characteristic so that the optimal cutoff frequency is obtained.
However, according to the conventional method above, the step of inputting a sine wave with a predetermined frequency and monitoring the output signal must be repeated for a plurality of frequencies, and there is then the drawback of the large amount of tuning work involved. Further, a high-cost analog signal generation circuit is required to supply a sine wave, which adds to the costs of a tester.